Delamination is the separation of a roofing material's bonded layers, most often used to describe plywood or OSB decking coming apart at its internal glue lines after prolonged exposure to trapped moisture. A delaminated panel loses much of its structural strength even while it may still look intact from below, since the separation starts between internal layers rather than on a visible surface. It can also describe laminated shingles losing their bond, though decking delamination is the more serious version, since it undermines the surface everything is fastened to.
Plywood and OSB roof decking are both manufactured by bonding multiple thin layers or strands of wood together under heat and pressure with a structural adhesive, and that bond is what gives the panel its strength as a single, solid sheet rather than a stack of loose wood pieces.
Delamination is what happens when that internal bond fails, most often because water has repeatedly soaked into the panel, commonly from a slow, undetected roof leak that has been feeding moisture into the same section of decking for months or years.
Water does not need to pool visibly to cause this. Repeated wetting and drying cycles, or simply sustained dampness trapped between layers with nowhere to evaporate, gradually breaks down the glue bond from the inside, well before any rot or visible damage shows on either face of the panel.
A delaminated section can still look structurally sound from below, which is what makes it a genuinely tricky problem to catch early. The layers are separating internally while the panel's outer surface remains intact, so the loss of strength is not something a quick glance from the attic reveals.
Once delaminated, a panel loses much of the strength that made it a single solid sheet in the first place, and it can flex, bounce, or feel spongy underfoot in a way solid decking does not, a physical sign that often gets discovered only when someone actually walks the roof or the attic.
Delamination is one of the reasons roof decking cannot be judged from the outside, and why a tear-off, when the old shingles and underlayment finally come off and the deck is fully exposed, is often the first honest look anyone gets at whether the wood underneath is still doing its job.
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